1. Field of the Invention
The present invention relates to a method for producing a toner, a toner, a two-component developer, a developing apparatus and an image forming apparatus.
2. Description of the Related Art
Office automation equipments have been remarkably developed in these days and in line with such development, there has been a wide spread of copiers, printers, facsimile machines, and the like machines which form images through the electrophotographic system. In general, an image is formed by way of a charging step, an exposing step, a developing step, a transferring step, a cleaning step, and a fixing step in an image forming apparatus which employs the electrophotographic system.
At the charging step, a surface of a photoreceptor serving as an image bearing member is evenly charged in a dark place. At the exposing step, the charged photoreceptor receives signal light derived from a document image, resulting in removal, of charges on the exposed part of the photoreceptor whose surface thus bears an electrostatic charge image (an electrostatic latent image). At the developing step, a toner is supplied to the electrostatic charge image on the surface of the photoreceptor, thereby forming a toner image (a visualized image). At the transferring step, a recording medium such as paper or sheet is brought into contact with the toner image on the surface of the photoreceptor, and the corona discharge is then generated toward the recording medium from one side thereof which is reverse to the side in contact with the toner image, to thereby provide the recording medium with charges of which polarity is opposite to that of charges of the toner, thus transferring the toner image onto the recording medium. At the fixing step, the toner image on the recording medium is fixed by means of heat, pressure and so on. At the cleaning step, the toner is collected which has not been transferred onto the recording medium and thus remains on the surface of the photoreceptor. Through the above steps, a desired image is formed by the image forming apparatus employing the electrophotographic system.
In recent years, with trend of speed-up of process and energy saving, it is desired that a toner used in an electrophotographic image forming apparatus can be fixed at low temperature. A method for adding crystalline polyester having low melting point is proposed as a method for producing a low-temperature fixable toner. The production method includes a dry process and a wet process. Particularly, a method for producing a toner by a wet process is energetically investigated. For example, seven production methods described in Japanese Unexamined Patent Publications JP-A 2001-305796, JP-A 2006-84843, JP-A 2007-248666, JP-A 2006-106679, JP-A 2006-265416, JP-A 2008-33057 and JP-A 2005-128176 are known.
A method for producing a toner having low melting point using crystalline polyester by a wet process leads to the problem that the crystalline polyester does not stably disperse in water, and elimination of the problem becomes the issue. The methods for producing a toner described in JP-A 2001-305796, JP-A 2006-84843, JP-A 2007-248666, JP-A 2006-106679, JP-A 2006-265416, JP-A 2008-33057 and JP-A 2005-128176 attend to overcome the issue by the respective different methods.
In the method described in JP-A 2001-305796, it is essential to use crystalline polyester containing a sulfo group having high dispersion stability, and in emulsifying, the crystalline polyester is dispersed in water using a surfactant. In the method described in JP-A 2006-84843, it is essential to use crystalline polyester containing a sulfo group, and the crystalline polyester is melted and forcibly emulsified in water. In the method described in JP-A 2007-248666, it is essential to use crystalline polyester containing a sulfo group. The crystalline polyester is dissolved in an organic solvent, and the resulting solution is mixed with water while applying shear force, thereby emulsifying the crystalline polyester.
The methods described in JP-A 2001-305796, JP-A 2006-84843 and JP-A 2007-248666 attend to stabilization of dispersion by using crystalline polyester containing a sulfo group. However, when a sulfo group is contained in a toner in large amount, the problem occurs that environmental dependency of charging characteristics is increased, that is, environmental characteristics are deteriorated. Therefore, the amount of the crystalline polyester used in a toner is limited. A method of conducting encapsulation with non-crystalline polyester or the like, without deteriorating environmental characteristics while enhancing dispersion stability is exemplified to overcome the problem. However, even though encapsulation is conducted, it is extremely difficult on production method to completely cover the crystalline polyester, and the essential problem of deterioration of environmental characteristics is not overcome.
The method described in JP-A 2006-106679 conducts emulsification by using a surfactant. The method described in JP-A 2006-265416 conducts emulsification by preparing crystalline polyester from a monomer dispersed in water by emulsion polymerization. Both methods do not essentially require to use crystalline polyester containing a sulfo group, but are essential to use a surfactant. Therefore, when a toner is produced, the surfactant is introduced into the toner, and this leads to the problem that environmental characteristics of the toner produced are deteriorated. Additionally, it is extremely difficult to remove the surfactant introduced into the toner produced.
The method described in JP-A 2008-33057 conducts phase inversion emulsification by mixing crystalline polyester and non-crystalline polyester and dissolving the resulting mixture in an organic solvent. This method increases dispersion stability by mixing the two polyesters and dissolving the mixture in an organic solvent. However, crystalline polyester particles and non-crystalline polyester particles are dispersed in an entangled state after phase inversion emulsification, and the inherent crystallizability of the crystalline polyester is not achieved.
In the method described in JP-A 2005-128176, crystalline polyester is heat melted and dispersed in an alkaline solution. This method does not require crystalline polyester containing a functional group which deteriorates environmental characteristics, and conducts emulsification without using a surfactant. However, the heat melting does not sufficiently resolve entanglement, of crystalline polyester particles with each other, and dispersion particle size is increased. As a result, dispersion stability may be decreased and particle size of a toner produced may be increased. Furthermore, condensation of crystalline polyester is induced, and molecular weight of polyester particle may be increased. Additionally, in the method of dispersion by heat melting, particle size distribution of dispersed particles becomes wide, and scattering causes in size of the toner produced. As a result, sufficiently high definition image cannot be formed.
Thus, attempts to overcome the problem in dispersibility in the method for producing a toner using crystalline polyester have been made, however, in the conventional methods, other problems are involved with resolution of the problem, and it can be hardly said that the problem is sufficiently solved.